This invention relates to a circuit including multiple series-pass transistors connected in parallel for coupling a DC voltage supply subject to over-voltage transients to a high current electrical load, and more particularly to a control circuit for balancing power dissipation in the transistors.
In a motor vehicle electrical system, over-voltage protection circuitry is needed to protect electrical loads from damage due to over-voltage transients that can occur during jump-starting and load-dump conditions. Although passive shunt suppression devices such as Zener diodes or MOVs can be used in light-duty applications, the transient over-voltage energy in heavy duty applications can be too high to clamp with shunt devices. In such cases, an active device such as a series-pass transistor can be used to couple the voltage supply to the loads, and the conduction of the transistor can be controlled to provide a minimum impedance coupling in normal operation and a controlled impedance coupling in the presence of over-voltage transients. Since the transistor must be capable of dissipating the over-voltage energy, two or more transistors can be connected in parallel when there is a potential for a large amount of over-voltage energy. However, it is likely that the over-voltage energy will not be evenly balanced among the parallel-connected transistors due to parameter tolerances, causing one transistor to become hotter than the others. While such imbalances tend to even out during normal low impedance operation, they can become greatly exaggerated during linear (controlled suppression) operation due to negative temperature coefficient effects. In the case of field-effect transistors (FETs), for example, the gate threshold voltage tends to decrease with increasing transistor temperature, causing the hottest FET to turn on even harder, leading to a condition commonly referred to as power-hogging. A similar effect occurs in bipolar transistors, where increases in temperature cause the base-emitter voltage threshold to decrease and the transistor gain to increase. While the imbalance may be mitigated to some extent by utilizing a negative feedback element such as a source or emitter resistor that reduces the conduction of the transistor(s) bearing an inordinate share of the power dissipation, the added resistance causes a handicap during normal low impedance operation when the on-resistance has to be minimized. Accordingly, what is needed is an improved control for parallel-connected series-pass over-voltage suppression transistors that ensures acceptable load sharing during linear (controlled suppression) operation, while retaining minimum pass-through impedance during normal operation.
The present invention is directed to an improved series-pass over-voltage protection circuit including multiple parallel-connected series-pass over-voltage suppression transistors coupling a DC voltage supply such as a motor vehicle storage battery to one or more high current electrical loads. During normal operation, all of the transistors are biased to the fully conductive/enhanced state to provide very low pass-through on-resistance. However, during linear (over-voltage suppression) operation, individual transistors are enabled in sequence at a frequency that is high relative to the thermal time constant of the transistors, and with a small amount of conduction overlap between successively enabled transistors. Sequentially enabling the transistors guarantees at least a minimum level of load sharing, and the overlap minimizes switching-related output current transients.